Input Specifications vs Output Specifications


Dean Suttling

March 11th, 2022
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A specification is an act of identifying something precisely; stating a precise requirement or a detailed description of the design and materials used to make something. Therefore, the image conjured is an extensive document with definite descriptions of the requirements. Still, there is also the option to define what outcomes are required and allow proposals to meet these outcomes.

The difference described here is between an input specification and an output specification.

If we look at the differences between the two, including advantages and disadvantages, you will be able to weigh up what best suits your requirements and how you wish to manage projects.

By way of an example, the description of an input specification across a highway maintenance contract could read as follows:

“The contractor shall ensure highways drainage maintenance is cleaned, gullies and their immediate pipe connection are emptied and cleansed, in rural areas, once per year; urban areas, once per year; high-speed dual carriageways, twice per year.”

The description of the works and the frequency by which you should do it is stated, and once tied to a range location of the existing assets, routine maintenance is established.

By way of an example, the description of an output specification for the same work type could read as follows:

“The contractor shall identify where persistent problems are identified with gully cleansing and their immediate pipework, being blocked by silt and detritus regularly, the gullies and pipe connections are to be specifically identified and programmed for a greater cleansing frequency by the contractor to ensure the system continues to function properly.”

The difference between the two is that with the input specification, the contractor carries out their work at the intervals stated across the network’s geography irrespective of existing condition or level of functionality. Alternatively, the output specification allows the contractor to establish a network condition and prioritise resources to make the most significant difference in the most affected areas in support of an operational network.

Input Specifications

Known as traditional specifications, they describe, sometimes in substantial detail, the exact services to be performed, including the standard that you should achieve and the methodology that you should adopt to earn the required standard. If there is a failure to follow the method or perform the necessary standard, those delivering the works would be considered in breach of contract; the works could be noted as defective and required to be undertaken again.

However, where clarity is absolute, and there is no room for ambiguity, then from a client’s perspective, the appeal is that you will know what you are getting at the outset of a contract with a level of certainty. The development of these specification types can be mainly involved when drafting them. It is the contractor’s responsibility to propose how they intend to achieve the standards to the client.

The input specification puts the onus on the contractor to demonstrate they can achieve the specification and then explain to the client that they have constructed the works to the desired standard. It imparts risk to the supply chain by way of the client not accepting the works and therefore restricting payment should the contractor be unable to meet the prescriptive requirements of the contract.

Advantages and Disadvantages

For advantages, the key one for some clients is the confidence in knowing exactly what they are buying and the knowledge that if they do not get precisely what the contract states, they have several options available to enforce to rectify the situation.

In terms of disadvantages, there is a heightened need for the client to create documents devoid of ambiguity as, if there is ambiguity, this could lead to extensive post-contract changes or if found at the bidding stage, it could lead to questions that undermine the bidding process. Equally, if they are uncovered post-contract, then issues, even if seemingly small in detail, can mushroom into disputes once scaled up over the duration and variability of the contract.

Also, if input specifications offer clarity in the extreme of clients, this may problematic. Being overly dictatorial and failing to involve the supply chain in developing the specification could be short-sighted as the very expert professionals and contractors that are sought to be employed are not consulted in the development and solution of the project needs.

Output Specifications

As noted in the example above, output specifications require the contractor to meet a contractual outcome. They meet this requirement at the contractor’s discretion, and therefore, some autonomy can be permitted to achieve this. The contractor will be asked to demonstrate through the bidding or tendering process how they intend to meet these outcomes, and it is for the client to review which bidder offers the most economical and realistic solution that would still meet these outcomes.

The content or structure of an output specification will need to contain the business or project objectives the function the building or project is to perform, which then feeds into the scope of services to be provided. This will likely include the number of users, e.g., a hospital with many beds, a school with many pupils, a housing development, a mix of property types, etc. You should provide details regarding the stakeholders and the constraints, including access, environmental, planning consents. Performance objectives are essential, too, such as the sustainability requirements to be met.

The difference in approach is that with an input specification, the client, at the tender return stage, is analysing rates and programmes as the ability of the contractor to deviate away from the input specification is at the client’s absolute discretion. Therefore, the contractor’s power to offer innovative solutions that may exceed the client’s requirements by way of price, time and quality are limited to alternative bid proposals. They are not afforded sufficient freedom to define how they could achieve the contract requirements and therefore is the client achieving total value for money through the contract price with an input specification in place.

The most intelligent approach is to consider the contracting strategy on a project-by-project basis depending on the building use and other factors. It may be that the best approach is an input specification due to the limited nature of the innovation available, or it may be that a prescriptive specification is needed due to the building’s exact purpose, and due to this, the client is not open or cannot be available to alternative proposals.

Advantages and Disadvantages

The main advantage of the output specification is that the level of innovation allowed the supply chain to consider more efficient ways to meet the project objectives. This allows the client to reduce the level of supervision and overarching control over the contractor they would typically provide on a project with an input specification. The risk of demonstrating the outcomes or being met now resides even more with the contractor.

It is not always beneficial operating on an outcome specification however, as imparting the control of the outcomes, means imparting more trust on the supply chain and without proper due diligence through the tender process, the solution offered to meet the outcomes may prove to be inadequate. Whilst this risk does sit with the contractor, responsibility for project failure does ultimately reside with the employer.

In such an instance, it can be difficult to assign responsibility as one party would likely argue that the method, they are imparting meets the outcome whilst the other party believes that it is inadequate. Without the detailed specification or description of the works needed, disputes can arise which are technically complex to resolve.

Output Specifications and Standards

Describing the output-based specification or the functional requirements allows innovation. As noted above, they do not describe in detail the flooring, doors, windows, door handles etc. there should still be a minimum standard to achieve, and indeed, if you are considering a programme of works or developments, then adopting a set of minimum standards against which you can apply the innovation is a pragmatic approach.

Many standards are UK Government led and therefore for expediency, and as a case to demonstrate value for money through innovation, output specifications are preferred. But to operate these, a set of minimum standards is used, the Common Minimum Standards (CMS), and their adoption is mandatory.

It is not exhaustive, the latest version is just 15 pages, but there is a reference to numerous other policies and procedures. Broadly it covers:

  • General Standards – a reference to the Construction Strategy including principles to enable better productivity, measuring client capability, and embedding digital construction and collaboration.
  • Project and Programme Procurement Standards – references to how procurement options and project strategy should take account of whole-life value for money and that procurement decisions should not just be made on capital cost.

Also, procurement routes should be limited to ECI, D&B and Prime Cost Contracting, Two Stage Open Book and Framework etc., which feeds into the above general standards around collaboration.

Also, suppliers are to be BIM Level 2 standard or provide a plan for achieving BIM Level 2. Furthermore, you should complete standards around prompt payment and risk assessments as part of the assurance reviews.

  • Health, Safety and Welfare Standards – references to how all clients should use CDM to assess the H&S performance of suppliers via a pre-qualification assessment process.

A requirement for all suppliers to be registered with a suitable site management scheme such as the Considerate Constructors Scheme. Alongside this, You should draft contracts such that all those attending sites or visiting sites should demonstrate competence for the task they are performing.

  • Design Standards – a reference to developing a clear project design brief that not only addresses current requirements in terms of physical and social requirements but future-proofing the project development or use by demonstrating how future requirements have been considered.

As noted above, stakeholders, including end-users, should develop the project brief and output specification, including how You will measure success.

  • Sustainability Standards – a reference to managing whole life carbon to capture information and minimise carbon output. On top of this, you will review the investment in line with best practices for apprenticeships.

Notwithstanding the above sustainability, minimum standards also need to consider buying standards that deal with environmental assessment processes such as BREEAM or equivalent.


If you are an experienced and competent client, capable of providing accurate and detailed specifications, and through this experience, you understand specifically what you need, then an input specification might be best suited to your needs.

If, however, you have experience but not necessarily in a specific area of construction, but you know what the function needs to be, then an outcome specification could be the best route so that you can invite innovation and encourage collaboration and engagement.

Therefore, it is a case of understanding where you are in light of the above and then using this knowledge to decide whether an input or output specification works best for you.

About Dean Suttling

A member of the Royal Institution of Chartered Surveyors, Dean has twenty years of experience in commercial management and quantity surveying, undertaking roles for contractors, clients, and consultants.

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